Trees are the ultimate carbon capture and storage machines. Large-scale tree planting is an increasingly popular component of global efforts to meet climate targets. But forests are complex ecosystems, and poorly planned planting efforts can actually increase the amount of carbon dioxide in the atmosphere and exacerbate global warming. How do we balance the clear need to plant more trees with the other demands on land use and how do we take account of the other benefits provided by increasing tree cover? Darren Moorcroft is CEO of the Woodland Trust, Clare Pillman is CEO of Natural Resources Wales and Rob Penn is a writer, broadcaster and photographer.